Various pump arrangements, for example those deployed in nuclear processing plants, require pumps that provide discharge at variable flow rates while maintaining essentially constant discharge pressure. These arrangements frequently employ vertical-axis cantilevered-shaft pumps. Such pumps are particularly well suited to nuclear processing plants, since they have no submerged bearings or stuffing boxes in the liquid or slurry to be pumped. Since these pumps are frequently located in radioactive areas and can not be readily serviced, the pumps must be able to operate at any point from shut-off to maximum specified flow rate, and utmost reliability is required.
Conventional centrifugal pumps can be operated at variable speeds to provide variable flow rates. However, conventional pumps are unsuitable for use in arrangements as described above, since their discharge pressures vary with the square of their speed ratios.
Simply operating a conventional centrifugal pump at constant speed and selectively throttling the discharge is also unsuitable, since the flow pattern through, and at the exit from, the impeller is significantly unstable, thus creating hydraulic forces that in turn cause severe vibration.
Furthermore, since low flow designs require smaller impeller openings, slurry in suspension tends to collect at the openings, and eventually obstructs or blocks pump discharge. This problem is exasperated if small pieces of solids are present in the slurry.
Finally, simply using a larger pump is also unsuitable, since operation of such pumps near the shut-off pump results in unstable hydraulic forces.
It is therefore apparent that the need exists for a vertical-axis cantilevered-shaft centrifugal pump that maintains a stable flow while providing variable discharge flow rates and maintaining a relatively constant discharge pressure.